'\" et
.TH SETPGID "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
setpgid
\(em set process group ID for job control
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
int setpgid(pid_t \fIpid\fP, pid_t \fIpgid\fP);
.fi
.SH DESCRIPTION
The
\fIsetpgid\fR()
function shall either join an existing process group or create a
new process group within the session of the calling process.
.P
The process group ID of a session leader shall not change.
.P
Upon successful completion, the process group ID of the process with
a process ID that matches
.IR pid
shall be set to
.IR pgid .
.P
As a special case, if
.IR pid
is 0, the process ID of the calling process shall be used. Also, if
.IR pgid
is 0, the process ID of the indicated process shall be used.
.SH "RETURN VALUE"
Upon successful completion,
\fIsetpgid\fR()
shall return 0; otherwise, \-1 shall be returned and
.IR errno
shall be set to indicate the error.
.SH ERRORS
The
\fIsetpgid\fR()
function shall fail if:
.TP
.BR EACCES
The value of the
.IR pid
argument matches the process ID of a child process of the calling
process and the child process has successfully executed one of the
.IR exec
functions.
.TP
.BR EINVAL
The value of the
.IR pgid
argument is less than 0, or is not a value supported by the
implementation.
.TP
.BR EPERM
The process indicated by the
.IR pid
argument is a session leader.
.TP
.BR EPERM
The value of the
.IR pid
argument matches the process ID of a child process of the calling
process and the child process is not in the same session as the calling
process.
.TP
.BR EPERM
The value of the
.IR pgid
argument is valid but does not match the process ID of the process
indicated by the
.IR pid
argument and there is no process with a process group ID that matches
the value of the
.IR pgid
argument in the same session as the calling process.
.TP
.BR ESRCH
The value of the
.IR pid
argument does not match the process ID of the calling process or of a
child process of the calling process.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
The
\fIsetpgid\fR()
function shall group processes together for the purpose of
signaling, placement in foreground or background,
and other job control actions.
.P
The
\fIsetpgid\fR()
function is similar to the
\fIsetpgrp\fR()
function of 4.2 BSD, except that 4.2 BSD allowed the specified new process
group to assume any value. This presents certain security problems and
is more
flexible than necessary to support job control.
.P
To provide tighter security,
\fIsetpgid\fR()
only allows the calling process to join a process group already in use
inside its session or create a new process group
whose process group ID was equal to its process ID.
.P
When a job control shell spawns a new job, the processes in the
job must be placed into a new process group via
\fIsetpgid\fR().
There are two timing constraints involved in this action:
.IP " 1." 4
The new process must be placed in the new process group before the
appropriate program is launched via one of the
.IR exec
functions.
.IP " 2." 4
The new process must be placed in the new process group before the
shell can correctly send signals to the new process group.
.P
To address these constraints, the following actions are performed. The
new processes call
\fIsetpgid\fR()
to alter their own process groups after
\fIfork\fR()
but before
.IR exec .
This satisfies the first constraint. Under 4.3 BSD, the second
constraint is satisfied by the synchronization property of
.IR vfork (\|);
that is, the shell is suspended until the child has completed the
.IR exec ,
thus ensuring that the child has completed the
\fIsetpgid\fR().
A new version of
\fIfork\fR()
with this same synchronization property was considered, but it was
decided instead to merely allow the parent shell process to adjust the
process group of its child processes via
\fIsetpgid\fR().
Both timing constraints are now satisfied by having both the parent
shell and the child attempt to adjust the process group of the child
process; it does not matter which succeeds first.
.P
Since it would be confusing to an application to have its process
group change after it began executing (that is, after
.IR exec ),
and because the child process would already have adjusted its process
group before this, the
.BR [EACCES] 
error was added to disallow this.
.P
One non-obvious use of
\fIsetpgid\fR()
is to allow a job control shell to return itself to its original
process group (the one in effect when the job control shell was
executed). A job control shell does this before returning control back
to its parent when it is terminating or suspending itself as a way of
restoring its job control ``state'' back to what its parent would
expect. (Note that the original process group of the job control shell
typically matches the process group of its parent, but this is not
necessarily always the case.)
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIexec\fR\^",
.IR "\fIgetpgrp\fR\^(\|)",
.IR "\fIsetsid\fR\^(\|)",
.IR "\fItcsetpgrp\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_types.h>\fP",
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
